President Barack Obama has called the Ebola outbreak in West Africa "a threat to global security", as he announced a larger US role in fighting the virus.

The world was looking to the US, Mr Obama said, but added that the outbreak required a "global response".

The measures announced included ordering 3,000 US troops to the region and building new healthcare facilities.

Ebola has killed 2,461 people this year, about half of those infected, the World Health Organization (WHO) said.

The announcement came as UN officials called the outbreak a health crisis "unparalleled in modern times".

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The WHO says the most urgent immediate need is more medical staff

The funds needed to fight the outbreak have increased 10-fold in the past month and $1bn (£614m) was needed to fight the outbreak, the UN's Ebola co-ordinator said.

'Massive surge needed'

Mr Obama said that among other measures, the US would:

Build 17 healthcare facilities, each with 100 beds and isolation spaces, in Liberia

Train as many as 500 health care workers a week

Develop an air bridge to get supplies into affected countries faster

Provide home health care kits to hundreds of thousands of households, including 50,000 that the US Agency for International Development (USAID) will deliver to Liberia this week.

Mr Obama called on other countries to step up their response, as a worsening outbreak would lead to "profound political, economic and security implications for all of us".

Ebola only spreads in close contact, and there is no cure and no vaccine. The outbreak began in Guinea last December before spreading to its neighbours Sierra Leone and Liberia.

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Media captionThe current outbreak is the deadliest since Ebola was discovered in 1976

Mr Obama said the outbreak had reached epidemic proportions in West Africa, as the disease "completely overwhelmed" hospitals and clinics and people were "literally dying on the streets".

Tulip Mazumdar, global health correspondent

Mr Obama announced the sort of help that the WHO, Medecins Sans Frontieres and others have been calling for for many weeks.

The promise of 3,000 American troops to help build treatment centres and train thousands of medical staff in Liberia will be a crucial boost to fighting this epidemic, though much more is needed.

The health systems of the three worst-affected countries are among the weakest in the world and can't deal with this massive outbreak alone.

Several months into this crisis, there are still very few isolation and treatment centres. It means that when infected, people who pluck up the courage to get help are often turned away from medical facilities because there simply isn't room or enough medical staff to treat them.

That means they end up going home and infecting others.

Meanwhile in Guinea, a team of health officials was attacked on Tuesday in a village they were visiting to raise awareness of the illness.

People in Wamey, in the south of the country, threw stones at the team, which included WHO and Red Cross representatives. At least 10 officials were hurt, and several who escaped into the bush are still missing.

This is not the first such incident. There have been many reports of people in the region saying they do not believe Ebola exists, or not co-operating with health authorities, fearing that a diagnosis means certain death.

In Sierra Leone, people are preparing for a three-day lockdown ordered by the government in an attempt to stop the spread of Ebola.

The BBC's Umaru Fofana in the capital Freetown says many residents are stocking up on food.

A number of aid agencies, including MSF, have criticised the lockdown, saying it would not help contain the virus.

Also on Tuesday, Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, told a US congressional committee that 10 volunteers in a study on an Ebola vaccine had shown no ill effects from an early stage trial.

Earlier, the WHO welcomed China's pledge to send a mobile laboratory team to Sierra Leone.

Cumulative deaths - up to 13 September

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Figures are occasionally revised down as suspect or probable cases are found to be unrelated to Ebola